Atypical measles syndrome has been reported extensively in the pediatric medical literature. However, the clinical picture in the adult is similar to that of many other diseases, making the diagnosis elusive. The case reported here was unusually morbid. The patient, a young man, had been in excellent health until the onset of a perplexing syndrome. When seen by the author, he had been ill for 1 week with chills, pharyngitis, and vomiting; later, a nonpruritic, maculopapular rash developed. Symptoms progressed to pneumonitis and hepatitis. A rubeola titer was obtained and was found to be considerably elevated. Because of the high titer and the fact that the patient had been immunized against measles in early childhood, the diagnosis was atypical measles syndrome. Two theories are offered to explain the pathogenesis of this disease.